Field of the Invention
This invention relates to an optical information reading system.
Description of the Related Art
In an optical information reading system such as an optical disk player, for example, the cut-off spatial frequency of a reproduction optical system is generally restricted to 2NA/.lambda., where NA represents the aperture number of an objective lens and .lambda. the wavelength of a laser. For this reason, information signal comprising higher spatial frequency components than 2NA/1 cannot be reproduced, so that the recording, density of an optical disk is limited by the cut-off spatial frequency.
For extending the cut-off spatial frequency toward the higher frequency side, an optical disk has been proposed to have a reflective layer therewith unitizing a material, the reflectivity of which depends on the intensity of an incident light. The optical transfer function (OTF) for reproducing such an optical disk is substantially the sum of the OTF of a conventional reproduction optical system and additional OTF by a change in the reflectivity in beam spots of the optical disk. Therefore, unless the change in the reflectivity of the reflective layer is large, high frequency components cannot be reproduced since the absolute value of the OTF on the high frequency side becomes extremely small.